It is common in many vehicles, as standard equipment or optional equipment, to include a vehicular navigation system. These systems determine the present location of a vehicle and based on destination information provided by a user—usually street number, name, city and state—these systems plot out a route and track the vehicle's progress along that route as the vehicle is being driven.
Virtually all such vehicular navigation systems rely on the signals received from Global Positioning System (GPS) satellites to determine initial vehicle location and to provide speed and direction derived from successive location readings to enable the vehicular navigation system to track the vehicle.
Clearly, such systems therefore rely on continuous or frequent GPS location information which, in turn, requires continuous or frequent reception of GPS satellite signals during the journey.
GPS satellites provide adequately accurate positioning information to support navigation plotting and tracking. However, under some circumstances, these signals may be blocked or impaired. For example, in urban areas, such as New York City, where streets may be bordered on both sides by very tall buildings, the satellite signals may be blocked. In areas where the vehicle is traveling on roads bordered by dense tree foliage, the signals may be impaired. For systems that rely on continuous or frequent GPS signal reception, this can cause disruption and inaccurate tracking.